The invention relates to the in situ, or field measurement of hydrogen atom permeation into the walls of pipelines and vessels containing process streams and, particularly, petroleum process streams.
Every corrosion process involves the two basic chemical reactions of oxidation and reduction. In the case of corrosion of carbon steel, the oxidative reaction that results in the destruction of the steel matrix can be represented as:
Fexc2x0xe2x86x92Fe+2+2exe2x88x92xe2x80x83xe2x80x83(I)
In many petrochemical environments, the concurrent reduction reaction is the formation of atomic hydrogen,
H++exe2x88x92xe2x86x92Hxc2x0xe2x80x83xe2x80x83(II)
Furthermore, in most chemical environments the atoms of hydrogen produced by (II) quickly undergo a reaction to form molecular hydrogen which, for the most part, mixes into the process environment and passes as dissolved gas:
2Hxc2x0xe2x86x92H2(g)xe2x80x83xe2x80x83(III)
In the typical cases of general corrosion, the combination reaction forming molecular hydrogen occurs virtually concurrently with the reduction of hydrogen ions to atomic hydrogen. However, there are some chemical environments in which the combination reaction of atomic hydrogen to molecular hydrogen is impeded, which results in a higher concentration or lifetime of individual hydrogen atoms at, or very near the surface of the steel. A chemical environment common in the oil industry which causes this phenomenon is one in which hydrogen sulfide gas is present at a concentration of parts per million or greater levels. The presence of hydrogen cyanide and arsenic in the process stream are also known to cause this type of hydrogen corrosion phenomena to occur.
The presence of hydrogen sulfide in oil and gas production process streams can result in potentially destructive corrosion-related phenomena denominated by the general term hydrogen damage.
Hydrogen damage is caused by the permeation of atomic hydrogen into susceptible steels. Molecular hydrogen can become trapped in defects in the steel when highly soluble and mobile atoms of hydrogen that are diffusing through the steel matrix combine. When the molecular hydrogen is trapped in a void in the steel, pressure builds up over time, leading to blisters and cracks. Such hydrogen-induced cracking can eventually result in the failure of the pipeline or vessel.
The presence of hydrogen sulfide increases the number of hydrogen atoms permeating into the steel and therefore increases the potential for hydrogen damage. The adequacy of the measures undertaken to control the hydrogen permeation rate and hydrogen damage in oil production facilities that are sour in nature (i.e., where hydrogen sulfide gas is present), is one of the most significant concerns in the industry.
Various control methods have been used in an effort to counteract the potential damage of atomic hydrogen. Steel alloys have been developed that are resistant or immune to hydrogen damage. These alloys are quite expensive and are not a cost-effective solution for all applications. It is also possible to remove or to reduce the concentration of the hydrogen sulfide that is the catalyst for hydrogen damage by subjecting the crude petroleum to additional processing steps. This so-called sweetening process is used in many locations by the oil industry, but it is not always a cost-effective choice. A third control method is the introduction of chemical inhibitors into the process stream at a very low concentration. Since atomic hydrogen damage is a phenomenon that takes place in the interior of the steel matrix, chemical inhibitors and sweetening processes offer an indirect means of controlling the hydrogen gas formation.
The problem for the field engineer is quite significant, since no matter what atomic hydrogen permeation control method is employed, there are limited means available to enable him to evaluate the effectiveness of the control method chosen.
Several techniques have been developed over the years to measure the amount of atomic hydrogen that permeates steel process piping and vessels. Each of these techniques has limitations that detract from its accuracy and/or usefulness under field conditions existing in the petroleum and petrochemical processing industry.
Various in situ hydrogen probes have been developed that can be inserted into process facilities to measure atomic hydrogen permeation rate by measuring the increase in pressure produced by molecular hydrogen. A typical example of this type of probe is the Model 6400 hydrogen probe manufactured by Rohrback Cosasco. The probe consists of a sealed chamber made of the process facility material that is inserted through a 2xe2x80x3 access fitting into the process stream. An external pressure gauge or transducer measures the day-to-day buildup of pressure inside of the sealed probe that results when the atoms of hydrogen that are permeating the probe reach the inner surface where they combine to form molecular hydrogen in the probe chamber. The major disadvantage to this type of device is that the signal-to-noise ratio for pressure measurement of molecular hydrogen buildup caused by atomic hydrogen permeation in a field process environment is low due to the fact that the flux is approximately 6E12 atoms of hydrogen per second per cm2, or less. A typical hydrogen probe such as the Cosasco 6400 has an effective probe surface area of approximately 42 cm2 and a minimum internal volume of approximately 20 cm3. This means that the measured pressure increase for this type of probe, under these conditions, will be less than 0.3 psi per day which is below the limit of detection for most pressure gauges or transducers. An additional consideration is the variation of internal pressure of a closed container due to temperature fluctuations. A ten degree variation in temperature will result in an observed pressure fluctuation of approximately 0.5 psi. It would not be possible to obtain reliable process information on a system at these levels at less than one week intervals.
Electrochemical measurement devices known as xe2x80x9cpatch probesxe2x80x9d are designed to be attached to the exterior of the vessel being monitored. Their method of operation assumes that atoms of hydrogen which enter into the steel from the inside will eventually find their way to the exterior wall where they can be measured. They suffer from several limitations.
First, if the metal of the process vessel is susceptible to hydrogen damage (it must be assumed that it is or there would be no need to monitor it), then a certain fraction of the hydrogen atoms that enter the steel matrix will remain trapped there as molecular hydrogen leading to hydrogen damage. The entrainment of this hydrogen gas will result in fewer atoms of hydrogen reaching the external surface of the process vessel and consequently a smaller and nonreproducible signal available from the patch probe.
Additional problems arise from the method of measurement, which is to oxidize the atoms of hydrogen as soon as they appear at the external surface of the process vessel walls. The oxidation is therefore being performed at the surface of the steel walls of the process vessel. First, it has been shown that the efficiency of oxidizing hydrogen atoms from the surface of steel in this environment is only 20%. Since the signals are small to begin with, an 80% signal loss is catastrophic. Secondly, the background current caused by the oxidation of steel itself is larger than the magnitude of signal measured from the oxidation of hydrogen atoms (at the 6E12 atoms per second per cm2 level). The signal-to-noise ratio is therefore less than poor. Some patch probe installations include machining the outer surface of the process vessel and then plating palladium on the surface. This has the potential of improving measurements by increasing the atomic hydrogen oxidation efficiency up to near 100% and eliminates the background signal due to oxidation of steel. However, not very many field engineers are willing to allow the external surfaces of their process vessels to be machined in order to apply a patch probe and palladium coating. A final problem with patch probes is the potential for caustic embrittlement that may be caused by the 0.1-0.2 N sodium hydroxide chemical environment that is in contact with the process vessel walls.
Another method used for the measurement of atomic hydrogen permeation is one in which an airtight seal is installed over an area of the external surface of the process vessel and the buildup in pressure due to the combination of atomic hydrogen to molecular hydrogen at the process vessel surface is measured over time. The method is similar to the in situ hydrogen probes discussed above, but there is no intrusion into the process vessel. One which is used quite extensively in the oil industry is sold under the brand name Beta Foil. The method offers the advantage of nonintrusive but suffers from the same limitations as the in situ hydrogen probes (including a severe temperature effect) and have not been found to be effective when hydrogen permeation rates are low, as they would be in a well controlled system. In addition, these devices are expensive to install and monitor.
Methods involving electrical measurements of small magnitude can be adversely affected by electrical noise signals in process streams created by pumps, cathodic protection systems, and other electrical equipment which can create electrical signals, in many cases, of equal or greater magnitude than that from the atomic hydrogen permeation being measured.
U.S. Pat. No. 5,405,513 discloses methods and apparatus for laboratory-based measurements in a test cell at, or near, ambient pressure and temperature. The method and apparatus disclosed lacks the capability for the in situ evaluation of actual process streams and vessels where the pressures can be as high as 3000 psi and temperatures reach 300xc2x0 F./150xc2x0 C., and therefore cannot be utilized for in situ field applications.
Since the potential for hydrogen damage in a given susceptible steel is directly proportional to the amount of atomic hydrogen diffusing into the steel matrix, a method and apparatus for quantitative measurement of atomic hydrogen diffusion in steel process vessels and pipelines would provide a valuable tool for evaluating the existence and rate of hydrogen damage and the methods undertaken for its control.
It is therefore an object of the invention to provide a method and apparatus for the in situ measurements that will provide a reliable indicator of the atomic hydrogen permeation rate in pipelines and vessels containing process feedstreams.
It is a further object of the invention to provide a method and apparatus for the in situ measurement of corrosion rates under the conditions described above.
Another object of the invention is to provide an electrochemical atomic hydrogen permeation measurement method that is electrically isolated from the process environment.
It is also an object of the invention to provide for such in situ measurements under field conditions of pressures ranging up to 3000 psi and temperatures ranging up to 300xc2x0 F., or greater.
It is another object of the invention to provide a hydrogen permeation detection probe that can be employed with existing industry standard two-inch access fittings, and further to provide a probe of variable length for use in pipelines and vessels requiring surface mounting and/or insertion of the probe into process streams.
Another important object of this invention is to provide a probe that possesses a high level of installation security and protection against leakage under field conditions prevailing in petrochemical processing facilities.
It is another important object of the invention to provide a probe and a method for its operation that is intrinsically safe and that presents no potential fire hazard.
Yet a further object of the invention is to provide a method and means for accomplishing the above objectives that includes the acquisition and storage of atomic hydrogen permeation data over extended periods of time, and its storage, as for example, on a magnetic recording media, or RAM (random access memory) device, and the downloading and transmission of such stored data into the memory of a general purpose computer at a remote location for subsequent processing and analysis.
The above objectives and other advantages are achieved in accordance with the apparatus and method of the invention by providing a probe for use in field installations for obtaining data in real time from which data the atomic hydrogen rate of permeation can be determined. The probe comprises an hydrogen damage-resistant metal test specimen electrode of planar configuration having a first face in contact with the process stream, and through which test specimen atomic hydrogen can pass without being trapped. The opposite side, or second face of the test specimen electrode, which is preferably plated with palladium is in contact with an electrolyte solution, such as sodium hydroxide (NaOH) contained in a sealed reservoir. A second counter electrode is positioned proximate the specimen electrode in the electrolyte reservoir, and a voltage potential is established between the electrodes. When atomic hydrogen reaches the palladium surface after permeating the test specimen electrode, it is ionized by the positively charged surface and an electrical current flows between the electrodes. In a preferred embodiment, an external embedded microcontroller is electrically connected to the electrodes. The associated embedded microcontroller monitors and measures the current flow. Data from the embedded microcontroller is stored in digital form and converted by appropriate circuitry into quantitative data that indicates the number of atoms migrating per unit of surface area of the pipeline or vessel.
The test specimen electrode is fabricated from a thin flat sheet of steel that is resistant to hydrogen damage so that hydrogen atoms pass through and are not trapped in the matrix of the steel. In a preferred embodiment, the thickness of the test specimen is in the range from about 0.010 to about 0.050 inches, with a preferred thickness being 0.032 inches. The preferred metal for the test specimen is A106 seamless carbon steel pipe in which a minimum of 10% of the pipe wall thickness has been machined away from both the inner and outer walls of the pipe.
The second, or reservoir side of the test specimen electrode is coated, e.g. by plating, with elemental palladium in order to maximize the atomic hydrogen oxidation efficiency, i.e., the oxidation of hydrogen atoms to hydrogen ions, H+, and to minimize the background current. The H+ ions react with the NaOH ions to form water. Palladium is preferred, since the metal coating significantly enhances the signal produced, as compared to an uncoated specimen. The palladium coating can be applied in as thin a molecular layer as possible, and a thickness of about one-ten thousandths (0.0001) of an inch has been found to perform satisfactorily.
The counter electrode is preferably formed from platinum to maximize the conductivity and measurement of the current flow and avoid any corrosion effect.
After the test specimen has been prepared, its weight is determined so that any change in weight after exposure can be measured. Thus, overall corrosion rate for the duration of the test exposure can be calculated, as well as the instantaneous permeation rate at any given time. Further details and specifications concerning the preparation of the test specimen electrode, as well as the utility and importance of this dual measurement capability is described in U.S. Pat. No. 5,405,513, the disclosure of which is incorporated herein in its entirety by reference.
A reservoir containing sodium hydroxide forms a part of the probe assembly and is in fluid communication with the permeation test specimen. Hydrogen atoms which have permeated the test sample are oxidized to hydrogen ions by the electrochemical conditions present in the cell. The sodium hydroxide in the reservoir then neutralizes the hydrogen ions. The hydrogen permeation rate is determined from the current produced from the electrochemical oxidation of hydrogen atoms. The corrosion rate, which is based on data taken from the corrosion test sample and associated electrodes, is determined by standard methods.
The entire probe assembly is constructed for use with industry-standard access devices that will permit fluid-tight installation at the test site. The probe assembly is isolated from the non-conductive materials to minimize background electrical effects, or noise.